The present invention relates to the production of metals, metalloids or their alloys by the use of self-reducing compacts containing carbon-reducible metal oxides and carbonaceous matter when the compacts are subsequently smelted in a submerged arc furnace. Examples of the products produced are: silicon, iron and ferroalloys.
It is known in the art to reduce iron ore in the form of lumps and pellets made from fine iron ores using hot carbon monoxide and hydrogen and to subsequently melt the resulting sponge iron in an electric arc furnace. It is also known in the art to mix relatively large pieces of iron ore, pellets of iron ore or sinters thereof with pieces of coke and coal and smelt such mixtures in a submerged arc furnace. Further it is known to make pellets from a mixture of fine chromite ore and fine particles of coke or anthracite coal with the incoproration of a binder and to partially reduce such pellets in a rotary kiln. The discharge from the kiln is later melted in an electric arc furnace to produce ferrochrome. The common method of producing ferroalloys is to feed a mixture of pieces or sinters of ore and pieces of coke or coal into a submerged arc furnace for smelting. In this practice the smaller particles of ore must be excluded from the feed material because such particles cause the formation of bridges in the furnace. When such bridges collapse an undesired eruption of gases is caused which results in a consequent variation in the composition of both the tapped metal and the slag. A principal disadvantage of the present conventional practice is that the reactants are separated, that is, the ore pieces melt and dissolve in the slag pool under the arcs, while the coke pieces which are required for metallization, float in the upper part of the slag containing the metal oxides. The present commercial process requires that a relatively slow heat consuming reaction between two separated phases. Further, if the length of the arcs is excessive, an excess of ore will be fused resulting in incomplete metallization. In such cases the slag will cool and thicken to such an extent that a satisfactory separation from the molten metal can not be made. Conversely, if the arcs are excessively short, an insufficient amount of ore will be fused resulting in a decreased production rate. In addition, the fusion rate of the ores is dependent on a number of factors which are difficult to control. One of the more important factors is the size distribution of the ore and the carbonaceous material which may vary widely making it difficult to initially achieve favorable production conditions and even more difficult to maintain favorable production conditions over long periods of time. A further disadvantage of the present commercial processes is that the ore pieces become sticky when the fusion process starts and tend to form bridges which prevent the descent of the charge and cause undesirable gas eruptions upon collapse. Furthermore, the mixture of ore pieces and coke pieces tend to segregate during handling and also during charging into the arc furnace. Such segregation further contributes to the undesired formation of bridges. Another harmful effect of the bridges is that they force the hot carbon monoxide gas, which is developed in the slag pool and carries a partial pressure of metal vapors, to vent through channels in the burden, whereby a beneficial condensation of the vapors in the burden is prevented or diminished.
The present invention provides a means to utilize fine ores, fine concentrates of ores and fine carbonaceous material as raw materials in the production of silicon, iron and iron alloys, such as, ferromanganese, ferrochrome, ferrosilicon and ferrochromesilicon, in a submerged arc furnace. The present invention provides for a smoother furnace operation, increased metal recovery, and a more constant and predictable composition of the metal or metalloid and the slag. The foregoing advantages are achieved by initially forming self-reducing compacts which contain carbon-reducible oxides of metals or metalloids and a carbonaceous reducant and, if required, a slag forming additive. Accordingly the compacts, preferably, serve as the sole feed material for smelting operations carried out using a submerged arc furnace. A particularly important advantage of the present self-reducing compacts is that metallization of iron oxide and a substantial part of other useful oxides takes place as the compacts are heated to temperatures below the temperature at which fusion begins.